Alternative hardmask materials have been developed to assist in transferring patterns from photoresist to underlying layers. Typically, the hardmask may be metal, silicon oxide or silicon nitride. However, some device structures already include metal, silicon oxide and/or silicon nitride layers to be patterned. It may not be desirable to pattern such device structures using a metal, silicon oxide or silicon nitride hardmask, since there may be little or no etch selectivity between the hardmask and the material to be etched. The process of removing the hardmask could also remove or damage underlying layers. A successful etch mask material ought to have etch selectivity relative to metal layers, silicon oxide layers and silicon nitride layers. Amorphous hydrogenated carbon is one such material which has been used as a hardmask for metals, silicon oxide or silicon nitride. Amorphous hydrogenated carbon, also referred to as amorphous carbon and denoted a-C:H, is essentially a carbon material with no long-range crystalline order. It is desirable to use a gas-phase deposition technique to produce amorphous carbon layers. Plasma enhanced chemical vapor deposition (PECVD) has been used to form amorphous carbon layers as an alternative to traditional techniques. In a typical PECVD process, a hydrocarbon source, such as a gas-phase hydrocarbon or vapors of a liquid-phase hydrocarbon that have been flowed with a carrier gas into a PECVD chamber. Plasma is then initiated in the chamber to form an amorphous carbon layer on the substrate. Amorphous carbon may form defects on patterned substrates which relate to a lack of planar post-deposition surface.
Techniques are needed for forming planar amorphous carbon layers to help avoid non-planar defect related yield loss.